White House declares that the government of Bashar Assad crossed a 'red line' with sarin gas in spring. Some U.S. lawmakers call for more forceful action.

WASHINGTON — The White House declared Thursday that Syria had crossed a "red line" by using chemical weapons in that country's civil war, and in response, U.S. officials said, President Obama had authorized sending arms to some rebel groups.

The arms will be provided to the rebel Supreme Military Council, an official said. The council is the military arm of an umbrella group that represents more moderate factions of the forces arrayed against the government of President Bashar Assad. White House officials would not comment on the decision to supply arms.

In a two-month investigation, U.S. intelligence agencies found that sarin, a potent nerve gas, was used near Damascus, the Syrian capital, and in the northern city of Aleppo, Deputy National Security Advisor Ben Rhodes told reporters. Attacks took place from March through May, at a time when the U.S. and its allies had highlighted the issue and Obama had warned that use of chemical weapons would prompt a U.S. response.

U.S. officials estimated that 100 to 150 people, perhaps more, were killed in the attacks.

Rhodes made it clear that Obama, who has been deeply reluctant to involve the U.S. military in Syria's

2-year-old civil war, will continue to move cautiously. Though the administration will increase its support for the rebels, Rhodes said, Obama is not planning to impose a no-fly zone in Syria, a step pushed by Sen. John McCain (R-Ariz.) and other advocates of more forceful U.S. action.

"People need to understand that not only are there huge costs associated with a no-fly zone, not only would

it be difficult to implement, but the notion that you can solve the very deeply rooted challenges on the ground in Syria from the air are not

immediately apparent," Rhodes said. In many parts of Syria, civilians and armed groups from opposing sides are closely mingled, making an air operation hard to implement, he said.

The administration intends to consult with the United Nations and allies, as well as Congress, before choosing how exactly to respond, Rhodes added. Obama plans to meet with allied leaders and with Russian President Vladimir Putin next week in Northern Ireland when he attends an annual economic summit.

The decision to lift the U.S. ban on supplying arms to the rebels comes as the Syrian government and its allies, particularly the Lebanon-based Hezbollah militia, have made strong gains. Recent reports suggest they are preparing for an offensive around Aleppo, which is divided between pro-Assad and rebel forces.

Though the decision by the U.S. carries considerable symbolic importance, its ability to significantly alter the power equation in Syria depends in part on the types and amounts of weapons. Providing only small arms, for example, might not be enough to tip the balance.

Moreover, it remains unclear how long it will take new aid to reach the rebels.

In April, U.S. officials announced $127 million in advanced nonlethal military aid for the rebels, including armored vehicles. The administration has not yet given Congress formal notification that it intends to send that materiel.

Some analysts, as well as opposition leaders, have said that rebel militias will need heavier antitank and antiaircraft weapons to push back pro-Assad forces. Administration officials have opposed sending those sorts of weapons, fearing they could easily find their way to terrorist groups. Several of the rebel militias have ties to Al Qaeda and other militant Islamic groups, officials say, and those groups have gained strength as the conflict has dragged on.

Dan Layman, a spokesman for the Syrian Support Group, an opposition group with offices in Washington, said Gen. Salim Idris, commander of the Supreme Military Council, urgently needs weapons to hold off government forces around Aleppo.

"We have been in contact with Gen. Idris in the past few days, and his requests are the same: communications equipment, antitank weaponry, antiaircraft weaponry, and enough small arms ammunition to help him defend Aleppo, which is about to be under siege by nearly 20,000 regime and regime-allied forces," Layman said.

McCain and Sen. Lindsey Graham (R-S.C.) urged the president to take more forceful action: "The president's 'red line' has been crossed. U.S. credibility is on the line," they said in a statement. "Now is not the time to merely take the next incremental step. Now is the time for more decisive actions."

Andrew Tabler, a senior fellow at the Washington Institute for Near East Policy and a supporter of arming the rebels, called the U.S. announcement disappointing.

"Given the urgency of

the situation, the question is whether this is the policy

response everyone was looking for. The answer is no," he said. "A red line, by definition, is military enforcement. We don't have that yet."